This application relates generally to fiber optic device manufacture and more specifically to techniques for efficiently soldering components such as metallized fiber to a metallized region on a substrate.
In the fabrication of fiber optic modules and the like, it is necessary to bond optical fibers to substrates, and it is common to solder a metallized fiber to a metallized region on a substrate to accomplish such bonding. A gold-tin (Au—Sn) alloy solder such as 80Au-20Sn is often used due to a number of desirable properties. For example, soldering with 80Au-20Sn may be fluxless, thereby eliminating the damage to optical properties due to flux residue. Also, 80Au-20Sn solder is the highest in mechanical strength and creep resistance among soft solders, so that optical alignment can be maintained over a long period. Further, pull test results from fiber metallizing vendors suggest that the interface between Au—Sn solder and metallized (with Ni/Au, for example) fiber is often stronger than that between, for example, eutectic Sn—Pb and metallized fiber.
However, soldering with 80Au-20Sn is challenging. The heating should be localized, as is often required in optical packaging. Since the melting temperature of 80Au-20Sn is relatively high (280° C.), a strong and localized heating mechanism is needed. Laser soldering and electric resist heating are two commonly used mechanisms.
Laser soldering suffers from the limitation that because solder metal reflects laser light, focusing the laser beam directly on solder does not transfer heat efficiently. Also, laser soldering requires an optical path, which is often difficult in many-layered or complicated optical devices.
In electrical resist soldering, a resistor pad is formed on the substrate at or near the bond site, and current is delivered to the resistor to provide the necessary heating. An often-encountered problem is mechanical failures (cracking or delamination) of the heating electric resist layer. Particularly, thick-film resistors are more vulnerable than thin-film resistors, because thick-film resistors contain more inherent defects. Other non-trivial issues in resist heating are the structure design of resistor pad and the parameter selection of the heating process.